1. Field of the Invention
This invention relates to a key telephone system and more particularly to an intercom calling apparatus in such a system.
2. Description of the Prior Art
In a conventional key telephone system, when a person receives a telephone call on a central office line (hereinafter called CO line or outside line) and is requested to call a third party, he must first put the call on hold by depressing the hold pushbutton in his key telephone set (or called subset), and thereupon call the designated third party by means of intercom calling.
Referring to FIG. 1, the above-mentioned conventional key telephone system will be explained. Let it be assumed that one person receives a telephone call on an outside line by depressing the pushbutton of switch LK1 and lifting up the handset in his subset, such that contacts with switch LK1 and switchhook HS1, HS2 are changed from the normal position to its opposite position. To call the designated third party by intercom, he must first depress a hold pushbutton to open the switch LK.sub.n, the call on outside line is held by a CO line control circuit through an inside line, which line control circuit usually performs the functions of line busy supervision, line holding control, and ringing detection. Then he must depress the intercom pushbutton to change switch LK.sub.n to its opposite position to call the third party. The intercom caller's voice is then transmitted to the amplifier D in a key service unit via an intercom speech path, and thence to the amplifier E of a loudspeaker SP in each key telephone set via a calling signal line. The loudspeakers SP of each key telephone set of the key telephone system then sound simultaneously to call the third party. Thus, the switch LK.sub.n is used not only for initiation of the above described intercom calling apparatus, but also during the transmittal of the intercom message.
The above-described prior art intercom calling apparatus in a key telephone system suffers from the following disadvantages:
(1) when a calling subscriber calls a called party over a CO line and requests the called party to call a third party by intercom during the conversation over the CO line, the called party must first put the subscriber on hold;
(2) the called party cannot originate the intercom call if the intercom line circuit is already in use, because the contacts of a relay A are operated when the called party answering detector in the key service unit detects the intercom speech; and,
(3) the called party must depress an intercom pushbutton to close the contacts of a switch for connecting the transmitter of his subset to the intercom speech path, in addition to depressing the hold pushbutton.